In electrostatographic imaging processes such as electrophotography and dielectric recording, developed images of polymeric toner powder are transferred electrostatically from one surface to another, for example, from a photoconductive, or dielectric surface to a receiving sheet of paper or plastic. This transfer is induced by the electrostatic attraction of charged toner particles from the first surface to the more strongly charged second surface. The electrostatic charging of the second surface (the receiving sheet) can be accomplished in various ways, such as by corona charging or by positioning the sheet between the first surface and an electrically biased pressure roller or plate. The strength of the field thus created causes the toner particles to transfer from the first surface, e.g., the photoconductor, to the second surface, e.g., the paper.
When a dry toner powder image is transferred electrostatically from one surface to another, certain defects can occur in the image. Defects, known as "hollow character", "halo", "mottle" and "flake" defects, can appear in the lines, alphanumeric characters or solid areas of the developed image. In the hollow character defect, the inner portions of the lines and alphanumeric characters contain less toner than the outer portions or not toner at all. Such defects are especially prevalent when the electrostatic transfer is accomplished by means of a biased pressure roller or plate.
To avoid the hollow character defect and related problems of image transfer, the addition of a low surface energy liquid such as silicone oil to dry toner compositions has been suggested by Jadwin et al in U.S. Pat. No. 4,517,272. In addition, U.S. Pat. No. 4,332,715 of Ona et al discloses the mixing of a vinyl resin with a small amount of a particular organopolysiloxane oil. According to the patent, these compositions were expected to be useful in toners for electrophotography but no indication is given of improvement in image transfer with such compositions.
In any event, although Jadwin et al disclose the improvement of image transfer by the use of silicone oils, it has been found that other problems occur with them. One is that the silicone liquids migrate from the toner and coat the carrier particles. This interferes with the triboelectric properties of the developer and leads to instability of the charge on the developer. As a result, the toner charge decreases and throw-off of toner increases. Another problem is that, as the silicone liquids exude from the toner binder, they aggregate as discrete particles on the toner particles in a non-uniform random distribution. This causes the toner image to be non-uniform. In addition, silicone liquids tend to leave an oil scum on photoconductive films.
Suggestions have also been made to incorporate other specific polysiloxane materials in toners, for purposes other than the improvement of image transfer. For instance, Japanese Patent No. 56-1060 of Noue et al, suggests that a toner composition having a binder composed of a particular silicon-containing copolymer resin and a silicon-free copolymer resin has good releasing properties with respect to rubber fixing rolls. French Patent No. 2,167,047 of Erhardt et al, discloses a toner composition comprising an A-B-A block copolymer wherein one of the sequences A and B is a hard amorphous polymer and the other is a soft amorphous or crystalline polymer. In one case, the hard polymer can be a styrene or a methylmethacrylate polymer and the soft polymer can be, among other things, a siloxane polymer. This composition is said to be pressure fixable.